1. Field of the Invention
The present invention relates to a coal feeding and/or processing system and, more particularly to a continuous coal extrusion apparatus and method.
2. Description of the Prior Art
The U.S. reserve of coal is about 3 trillion tons. Although coal represents the most abundant fossil fuel in the United States, and in other countries, current consumption patterns indicate that petroleum-derived fuels represent 80% of the total U.S. consumption, and coal less than 20% of the total. A principal reason for the failure to utilize this vast coal reserve has been the lack of an economical process for transferring coal in a form suitable for efficient conversion. Whereas, petroleum-derived raw materials are readily pumped and transferred at any desired process temperature and pressure for rapid conversion, it has not been possible to move and process massive quantities of coal in a regulated and reliable manner through conventional fuel processing reactors.
The lack of high flow rates, and lack of capability of adjusting total feed rates to match particular requirements of known conversion schemes has severely limited coal process development and means for transferring coal, or other carbonaceous material from the solid state at ambient pressures, directly into elevated temperature and pressure vessels for continuous conversion. An effective direct coal transfer apparatus and method has long been desired, permitting use of raw coal directly at any desired flow rate and at any pressure. Fusion of coal in the open air by application of heat is described in coal analysis publications, for example, a 1971 publication by the Bureau of Mines, by Wu et al, Bulletin 661, entitled "Coal Composition, Coal Plasticity and Coke Strength." When the temperature of a small mass of coal is raised slowly, gases and vapors evolve until the softening temperature of the particular coal is reached. Typically, coal does not fuse at well-defined temperatures, but for each type of coal there is a short temperature range within which enough liquid product is produced to cause the entire mass of coal or of coal particles to coalesce, fusing sufficiently to become plastic. This phenomenon has been used in coal processing apparatus and methods to a limited extent, but in every reported proposed application, has led to enormously troublesome operating difficulties and a commercially unacceptable process probably due to the fact that as coal is maintained at fusion temperature the viscosity at first drops and then increases until tar and solids are formed which deposit on the apparatus.
Prior art literature is replete with references to specific clogging and setting difficulties whenever raw coal is utilized as a feed, in an elevated and pressure system. For example, it is disclosed in U.S. Pat. No. 2,519,340 at Column 1, that when raw bituminous coal is directly passed into a thermal conversion process vessel, portions of the coal fuse into a high viscosity melted tarry mass. The presence of such tars in the described fuel bed gradually fills the voids and thus prevents requisite free flow of combustion air and steam. It is further disclosed that mechanical unclogging and stoking does not provide a solution of this clogging problem, which arises in nonpressured feed coal fusion systems.
More recently, it has been disclosed in the April 1976 paper of A. H. Furman entitled "Pressurized Feeding on the Gegas System" presented at the 81st AlCHE Conference, Kansas City, Mo. that the direct heating and softening of coal at about 750.degree. F. in an electrically heated, vented plastic extruder apparatus of a stated type provided tarry consistency solid products and gas evolution during softening regarded as so uncontrollable that further direct extrusion experiments were discontinued, and coal feeding was effected by mixing the coal with a binder-lubricant such as coal tar or asphalt and extruding the mixture at a low temperature of about 200.degree. F. which is well below the softening point of coal.
Historically, there have been many systems for feeding coal. These have ranged from modifications of the primitive shovel to the large complex coal-feeding systems used in synthetic fuel plants. With the increasing interest in the production of synthetic gas and oil from coal, coupled with the economic advantages of large high-pressure gasifiers, the problem of reliably feeding coal continuously into pressure vessels at a high rate has become more acute. For several years, this has been recognized as a serious technical constraint in the commercialization of synthetic fuel plants.
There are several techniques for feeding coal into reactors that operate near ambient pressure but the choices narrow quickly as the pressure increases. Lock-hoppers have been used almost exclusively for pressures approaching 500 psi. Development work is under way to extend this range to approximately 1000 psi, even though at these pressures, the energy requirements for gas compression become large. Furthermore, lock-hoppers are limited to use with openings having maximum diameters of about 12-13 feet, do not handle fines too well, and it is difficult to reseal the hopper opening during and between loading cycles. Significant improvements in the lock-hopper techniques are needed to achieve the economic and reliability requirements of advanced processes currently under development. One variation of the lock-hopper method, a piston-feeder technique, is reported to significantly reduce the gas pressurization energy requirements while extending the pressure range to 1000 psi.
Above 1000 psi, the only commercially available technique is the slurry-pumping method where pulverized coal is mixed in approximately equal portions with water or a light oil and pumped by some form of positive displacement pump into the pressure vessel. This approach requires that the carrier liquid be separated from the coal at high pressure (except in liquefaction processes) thus placing added requirements for equipment and energy on the process. Other innovations have been advanced for feeding coal into pressures lower than 1000 psi, such as the paste-feed method, but these generally cannot be extended to higher pressure applications.